1. Technical Field
The present invention relates to a discharge lamp operating apparatus. Specifically, the invention relates to a discharge lamp operating apparatus used as a light source for a projector in movie theaters.
2. Background Art
In recent years, in movie theaters, replacement from film projectors to digital projectors is accelerated. Although a discharge lamp such as a short-arc type xenon discharge lamp is widely used as a light source both in film projectors and digital projectors, luminance required for the discharge lamp is different between the film projectors and the digital projectors. More specifically, in the digital projectors, focusing light into a narrower area is required in comparison with that required for the film projectors when focusing light on an end surface of an integrator rod. Therefore, noble gas (xenon) is charged into the discharge lamp for the digital projectors at a substantially higher pressure than the case of the discharge lamp for the film projectors in order to form a shorter and thinner arc.
In the discharge lamp having a higher noble gas pre-charge pressure, the lamp current density during lighting is high. Accordingly, the temperature of a distal end of the electrode is increased to a substantially high temperature, so that the electrode substance at the distal end portion of the electrode is subject to evaporation.
In the xenon discharge lamp, halogen is not included as a charged substance and hence a function to maintain the distal end portion of the electrode by an accumulation of the evaporated electrode substance on the distal end of the electrode by a so-called halogen cycle is not provided. Therefore, the distance between the electrodes is increased as the operating time of the discharge lamp advances, and the lamp voltage is increased accordingly and, consequently, a retention ability of the arc is lowered.
The discharge lamp is practically in operation in the horizontal posture in the interior of the projector, and hence xenon gas as light-emitting gas flows convectively in the light-emitting tube by heat generating while the discharge lamp is in operation. Therefore, as a result that an upward stress is imparted on the ark by the xenon gas flowing convectively, a phenomenon of the arc curving on the upper side or an upward displacement of an original point of the ark occurs. When such the phenomenon of the arc curving on the upper side occurs, the length of the discharge path of the arc is increased, whereby the lamp voltage required for maintaining the arc is increased. When the phenomenon of the arc curving on the upper side occurs excessively, the lamp voltage is increased excessively, and hence the discharge lamp goes out. The phenomenon of the arc curving on the upper side occurs more remarkably as the retention ability of the arc is lowered with an increase in interelectrode spacing of the discharge lamp.
In order to solve the problem as described above, a discharge lamp operation circuit having a method of applying a magnetic field for suppressing the phenomenon of the arc formed in the discharge lamp curving on the upper side is proposed (See JP-A-2011-187255).
According to the discharge lamp operating apparatus, a Lorentz force is applied downward by applying a magnetic field in a direction vertical to a direction of extension of the arc and in the horizontal direction with respect to the arc of the discharge lamp, and the Lorentz force suppresses the phenomenon of the arc curving on the upper side.
Therefore, the discharge lamp operating apparatus described above has a problem that the arc is deformed downward and becomes unstable when the magnetic field is applied on the arc when the phenomenon of the arc curving on the upper side does not occur on the arc in the discharge lamp, that is, when the arc is maintained in a horizontal stable state. Therefore, control of the magnetic field applying unit depending on the existence of the phenomenon of the arc curving on the upper side is required.
As means for confirming the occurrence of the phenomenon of the arc curving on the upper side, (1) a means for confirming the occurrence of the phenomenon of the arc curving on the upper side by picking up of an image of the ark by, for example, a CCD camera or the like and analyzing an obtained video data, and (2) a means for confirming the occurrence of the phenomenon of the arc curving on the upper side by sensing an increase in voltage of the discharge lamp generated in association with the occurrence of the phenomenon of the arc curving on the upper side are conceivable.
From the two means described above, the means (1) has problems that the entire operating circuit becomes complex as a whole, the cost of the operating circuit is high, and monitoring all the time while the discharge lamp is in operation is difficult. Therefore, the means (2) is practical.
However, the means for confirming the occurrence of the phenomenon of the arc curving on the upper side by sensing the increase in voltage of the discharge lamp has a following problem.
Since the current density of the discharge lamp used in the digital projector is extremely high during operation, a plurality of minute projections are liable to be formed at the distal end of the electrode by fusing of the electrode substance, and hence a flicker occurs because of transfer of the original point of the arc between the minute projections. When the flicker as described above occurs in the discharge lamp, a significant voltage change occurs in an extremely short time. When a magnitude of the voltage change caused by the flicker is analogous to a magnitude of a voltage change caused by the phenomenon of the arc curving on the upper side, the voltage change caused by the flicker is erroneously sensed to be a voltage change caused by the phenomenon of the arc curving on the upper side. Consequently, a magnetic field is applied on the ark by the magnetic field applying unit, and hence instability of the ark is further increased, whereby the discharge lamp goes out.